Gradient refractive index (GRIN) optics are well known and are the subject of recent reviews. In a conventional lens, an incoming light ray is refracted when it enters the shaped lens surface because of the abrupt change of the refractive index from air to the homogeneous material. The surface shape of the lens determines the focusing and imaging properties of the lens.
In a gradient refractive index (GRIN) lens there is a continuous variation of the refractive index within the lens material. In a simple GRIN lens, plane optical surfaces can be used. The light rays are continuously bent within the lens. The focusing properties are determined by the variation of refractive index within the lens material. There are two gradient index (GRIN) lens types described in the literature: axial gradient and radial/cylindrical/spherical gradient. In the axial gradient, the refractive index varies in a continuous way along the optical axis of the inhomogeneous medium. In the axial gradient, the surfaces of constant index are planes perpendicular to the optical axis. In the radial/cylindrical/spherical the index profile varies continuously from the optical axis to the periphery along the transverse direction in such a way that the surfaces of constant index are concentric spheres or cylinders about the optical axis.
The simple geometry of a GRIN lens with flat surfaces allows the efficient production and simplified assembly of systems of lenses. Varying the thickness of the lens can vary the lens parameters such as the focal length and working distance. Thin lenses down to 0.02 mm in thickness are possible. Alternatively, spherical, ball-like, GRIN lenses allow an image plane to be made to lie directly on the exit surface of the lens.
A conventional lens with spherical surfaces and with a homogeneous index of refraction will not focus light perfectly; there will be spherical and chromatic aberrations. It is also well known in the art that these aberrations can be reduced or eliminated by employing axial gradient lens blanks. An axial gradient lens is a lens that has an index of refraction profile that varies in one direction only, usually chosen to be the optical axis. These aberration free lenses can be used advantageously in a variety of optical systems, such as slide projectors, cameras, binoculars, and many other imaging devices. The number of lens elements required for a given task can be reduced as well as the weight and complexity of the system.
Nanolayered coextrusion of polymer materials has been previously described for production of transparent films of a tailorable refractive index. A subsequent stacking and consolidation of these nanolayered films has been shown to produce polymer parts with a designable internal refractive index distribution in two or three part directions. A requirement for the previously described GRIN objects to perform as optics or lenses is fabrication of highly transparent optical parts. Internal scatter inclusions in the GRIN objects resulting from internal structure of the nanolayered film or deriving from the bonding of individual nanolayered films into a gradient refractive index sheet can result in a loss of GRIN optic functionality.
U.S. Pat. No. 5,262,896 describes the fabrication of axial gradient lenses by the controlled diffusion process. The blanks for the fabrication of such gradient lenses can be made by a variety of processes, such as SOL-GEL, infusion, and diffusion and may be glass, plastic or other suitable optical material.
U.S. Pat. No. 4,956,000 describes a method and apparatus for fabricating a lens having a radially non-uniform but axially symmetrical distribution of lens material, in which the lens size and shape is determined by the selective direction and condensation of vaporized lens material onto a substrate.
U.S. Pat. No. 5,236,486 describes the forming of a cylindrical or spherical gradient lens blank from an axial gradient lens blank by heat molding (slumping). This process produces a monolithic lens with a continuous index of refraction profile.
U.S. Pat. No. 7,002,754 describes a hierarchically multilayered polymer composite for graded index of refraction (GRIN) lenses and a method to fabricate the same.